A "flowmeter" is a device used to measure the flow of fluid material through a defined area, such as the cross sectional area of a pipeline. The "flow" is the motion of the fluid, which may be liquid or gas or a combination of both. The flowmeter measures the flow in terms of the flow rate, the amount of the fluid that flows over a given period.
There are various ways to express flow rate, such as by mass flow rate, volumetric flow rate, or velocity of flow. Mass flow rate is the flow rate in units of mass per unit time, i.e., kilograms per second. Volumetric flow rate is the flow rate in units of volume per unit time, i.e., cubic meters per second. Velocity of flow is in units of length per unit time, i.e., meters per second.
Most measurements of flow are in terms of volumetric flow rate. Mass flow rate can be calculated from this measurement, although variations in pressure, density, temperature must be taken into account, particularly for gases. Similarly, a measurement of velocity of flow can be used to calculate mass or volumetric flow rate.
Conventional flowmeters are designed for single phase fluids, that is, fluids that are either gas or liquid. Some existing flowmeters are mechanical, where the flowing fluid displaces or rotates a solid body. The displacement or rotation is proportional to the flow rate. Another type of flowmeter is a differential pressure flowmeter, in which fluid is forced through some type of restricted area. This causes its velocity to change, causing a pressure difference that is proportional to flow. By measuring the pressure difference, such as with a differential pressure transducer, the flow rate can be determined. Other flowmeter types are thermal, electromagnetic, vortex generating, and ultrasonic. The criolis force flowmeter is widely used for measuring mass flow.
When the fluid whose flow rate is to be measured is a multiphase fluid, special problems arise. An example of such a fluid is a hydrocarbon fluid, which is typically a mixture of oil and gas as well as water. For a multiphase fluid, there is often a need to know the liquid and gas "cuts", that is, the fractional amount of each constituent at a given point in a flowline, as well as their rates. In the case of petroleum fluids, there is a need to know the oil cut as distinguished from both the water and gas cuts.
One consideration when measuring the flow rate of multiphase liquids is that the gas component tends to flow at a higher velocity than the liquid component. It is therefore necessary to separately measure the gas and liquid flow velocities or to measure the total flow velocity after mixing the fluid.
An additional consideration in measuring a multiphase fluid is that the density of the gas, except at very high pressures, is low compared to that of the liquid. This makes direct measurement of the gas fraction difficult. Typically, the liquid fraction is measured and the remainder is assumed to be gas. In other words, if a pipe section is half filled with liquid, then the other half is assumed to be gas.
The conventional approach to measuring multiphase flow rates is to separate the fluid into its constituents. This permits conventional single phase metering techniques. However, especially in the petroleum industry, as the water and gas content of recoverable petroleum has increased and oil fields have become more inaccessible, there is a need for more sophisticated multiphase flowmetering equipment.
Several patents have been issued that describe the use of nuclear magnetic resonance (NMR) analysis to analyze fluid flows that are not necessarily multiphase. These include U.S. Pat. No. 4,531,093, to Rollwitz, et al., entitled "Method and Apparatus for Coal Analysis and Flow Measurement"; U.S. Pat. No. 4,536,711, to King, et al., entitled "Method and Apparatus for Measuring Flow in a Pipe or Conduit"; and U.S. Pat. No. 4,866,385, to Reichwein, entitled "Consistency Measuring Device".
NMR techniques have been specifically applied to analyzing multiphase fluids. U.S. Pat. No. 4,785,245, entitled "Rapid Pulse NMR Cut Meter," describes a flowmeter that uses NMR analysis to determine the fraction of one component of a multiphase fluid flowing in a pipeline. The amplitude of the NMR signal from a desired component is compared to a reference signal representing a 100% sample of the component.